tree-optimization/44794 - avoid excessive RTL unrolling on epilogues

The following adjusts tree_[transform_and_]unroll_loop to set an
upper bound on the number of iterations on the epilogue loop it
creates.  For the testcase at hand which involves array prefetching
this avoids applying RTL unrolling to them when -funroll-loops is
specified.

Other users of this API includes predictive commoning and
unroll-and-jam.

	PR tree-optimization/44794
	* tree-ssa-loop-manip.cc (tree_transform_and_unroll_loop):
	If an epilogue loop is required set its iteration upper bound.